Load driving device, refrigeration cycle applicable apparatus, and air conditioner

ABSTRACT

A load driving device includes a smoothing capacitor, an inverter, and a control unit. The inverter includes two legs, each including upper and lower arm switching elements connected in series and converts direct-current power stored in the smoothing capacitor into alternating-current power. The control unit performs voltage drop prevention control for preventing the voltage across the smoothing capacitor from becoming a negative voltage. The control unit stops power running control on the load when the capacitor voltage is higher than the sum of a first voltage and a second voltage. The first voltage is a potential difference between a second terminal and a first terminal in the upper-arm. The second voltage is a potential difference between a second terminal and a first terminal in the lower-arm in the same leg as the leg of the upper-arm.

CROSS REFERENCE TO RELATED APPLICATION

This application is a U.S. national stage application of InternationalApplication No. PCT/JP2018/029777 filed on Aug. 8, 2018, the contents ofwhich are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a load driving device for driving aload such as a motor, to a refrigeration cycle applicable apparatusincluding the load driving device, and to an air conditioner includingthe refrigeration cycle applicable apparatus.

BACKGROUND

A load driving device generally has a smoothing capacitor providedtherein. Patent Literature 1 listed below describes that, duringoperation of the load driving device, the smoothing capacitor may benegatively charged, that is, the voltage across the smoothing capacitormay become a negative voltage.

A negative voltage of the smoothing capacitor may have an adverse effectsuch as a reduction in the life of the smoothing capacitor, which may inturn cause malfunction in the device coupled to the smoothing capacitor.Patent Literature 1 describes that a diode is connected in inverseparallel between both ends of the smoothing capacitor, and thereby theamount of application of the negative voltage of the smoothing capacitoris controlled not to exceed a value equivalent to a forward voltage dropacross the diode.

CITATION LIST Patent Literature

-   Patent Literature 1: Japanese Patent Application Laid-open No.    2013-240274

However, the technique in Patent Literature 1 requiring the diode to becoupled between both ends of the smoothing capacitor presents a problemin that the number of components is increased. In addition, thetechnique in Patent Literature 1 corresponds to a technique forcontrolling the amount of application of the negative voltage of thesmoothing capacitor not to exceed the value equivalent to the forwardvoltage drop across the diode, in which the voltage across the smoothingcapacitor is permitted to become some negative voltage. That is, thetechnique in Patent Literature 1 is not worth a technique for preventingthe voltage across the smoothing capacitor from becoming a negativevoltage.

The present invention has been made in view of the foregoingcircumstances, and an object thereof is to provide a load driving devicecapable of preventing the voltage across the smoothing capacitor frombecoming a negative voltage without setting up an additional component.

SUMMARY

In order to solve the above-mentioned problem and achieve the object,the present invention provides a load driving device supplyingalternating-current power to a load and driving the load, the loaddriving device comprising: a smoothing capacitor; an inverter having atleast two legs, each of the legs having an upper-arm switching elementand a lower-arm switching element connected in series with each other,the inverter converting direct-current power stored in the smoothingcapacitor into the alternating-current power; and a control unitcontrolling the inverter and performing voltage drop prevention controlfor preventing a voltage across the smoothing capacitor from becoming anegative voltage, Wherein the control unit stops power running controlon the load in a state in which the voltage across the smoothingcapacitor is higher than a sum of a first voltage and a second voltage,the first voltage being an electrical potential difference between asecond terminal of the upper-arm switching element and a first terminalof the upper-arm switching element with reference to the first terminalof the upper-arm switching element, the second voltage being anelectrical potential difference between a second terminal of thelower-arm switching element and a first terminal of the lower-armswitching element with reference to the first terminal of the lower-armswitching element in the same leg as a leg of the upper-arm switchingelement.

A load driving device according to the present invention provides anadvantageous effect that the voltage across the smoothing capacitor canbe prevented from becoming a negative voltage without setting up anadditional component.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a circuit diagram illustrating a configuration example of aload driving device according to a first embodiment.

FIG. 2 is a partial enlarged view of a rectification unit illustrated inFIG. 1 .

FIG. 3 is a partial enlarged view of an inverter illustrated in FIG. 1 .

FIG. 4 is a block diagram illustrating a detailed configuration of acontrol unit illustrated in FIG. 1 .

FIG. 5 is a block diagram illustrating an example of a hardwareconfiguration by which the functionality of a control system of thecontrol unit in the first embodiment is implemented.

FIG. 6 is a block diagram illustrating another example of a hardwareconfiguration by which the functionality of a control system of thecontrol unit in the first embodiment is implemented.

FIG. 7 is a time chart used for describing an operation at the time ofloss of power in the load driving device of FIG. 1 .

FIG. 8 is a graph illustrating various kinds of waveform examples whenthe operation illustrated in FIG. 7 is carried out.

FIG. 9 is a partial enlarged view of a section indicated by adouble-headed arrow in FIG. 8 .

FIG. 10 is a diagram used for describing a mechanism of occurrence of anegative voltage, which is considered problematic in the firstembodiment.

FIG. 11 is a graph for describing an influence on a smoothing capacitorillustrated in FIG. 1 when a negative voltage is generated.

FIG. 12 is a diagram for describing an influence on a control powersupply generation unfit illustrated in FIG. 1 when a negative voltage isgenerated.

FIG. 13 is a flowchart used for describing an operation performed involtage drop prevention control in the first embodiment.

FIG. 14 is a block diagram illustrating a configuration example of avoltage drop prevention control unit in the first embodiment.

FIG. 15 is a first time chart used for describing a circuit operation ofthe voltage drop prevention control unit illustrated in FIG. 14 .

FIG. 16 is a second time chart used for describing a circuit operationof the voltage drop prevention control unit illustrated in FIG. 14 .

FIG. 17 is a diagram used for describing current paths when the voltagedrop prevention control unit performs the operation illustrated in FIG.15 .

FIG. 18 is a diagram used for describing current paths when the voltagedrop prevention control unit performs the operation illustrated in FIG.16 .

FIG. 19 is a circuit diagram illustrating a configuration example of aload driving device according to a second embodiment.

DETAILED DESCRIPTION

A load driving device, a refrigeration cycle applicable apparatus, andan air conditioner according to embodiments of the present inventionwill be described in detail below with reference to the accompanyingdrawings. Note that the following embodiments are not intended to limitthe scope of the present invention.

First Embodiment

FIG. 1 is a circuit diagram illustrating a configuration example of aload driving device 100 according to a first embodiment. FIG. 2 is apartial enlarged view of a rectification unit 20 illustrated in FIG. 1 .FIG. 3 is a partial enlarged view of an inverter 40 illustrated in FIG.1 .

The load driving device 100 according to the first embodiment isconfigured, as illustrated in FIG. 1 , to temporarily convert an ACvoltage outputted from an AC power supply 10 that is a single-phasepower supply into a DC voltage, reconvert the DC voltage to an ACvoltage in the load driving device 100, and drive a permanent magnetsynchronous motor 50 that is an example of load. The permanent magnetsynchronous motor 50 is applicable to a compressor motor equipped in arefrigeration cycle applicable apparatus.

The load driving device 100 includes, as illustrated in FIG. 1 , therectification unit 20, a smoothing capacitor 30, a voltage detector 80,a control power supply generation unit 90, the inverter 40, a controlunit 60, and a current detector 82. The rectification unit 20 and theinverter 40 are electrically coupled to each other by DC bus lines 25 aand 25 b. The smoothing capacitor 30 and the control power supplygeneration unit 90 are each coupled between the DC bus line 25 a havinga higher electric potential and the DC bus line 25 b having a lowerelectric potential.

The rectification unit 20 rectifies the AC voltage outputted from the ACpower supply 10 to convert the AC voltage into a DC voltage. The ACvoltage and the DC voltage may also be reworded to be “AC power” and “DCpower”, respectively.

The rectification unit 20 includes, as illustrated in FIG. 2 , a leg 20Ahaving an upper-arm element UCP and a lower-arm element UCN coupled inseries with each other, and a leg 20B having an upper-arm element VCPand a lower-arm element VCN coupled in series with each other. The leg20A and the leg 20B are coupled in parallel with each other.

FIG. 2 illustrates, by way of example, a case in which the upper-armelements UCP and VCP and the lower-arm elements UCN and VCN are each ametal-oxide-semiconductor field-effect transistor (MOSFET). Theupper-arm element UCP includes a transistor 20 a and a diode 20 bconnected in inverse parallel with the transistor 20 a. The other ones,i.e., the upper-arm element VCP and the lower-arm elements UCN and VCN,are each configured similarly to the upper-arm element UCP. The term“inverse parallel” as used here means that a first terminalcorresponding to a source of a MOSFET is connected with an anode of adiode, while a second terminal corresponding to a drain of the MOSFETconnected with a cathode of the diode.

In the case in which the transistors 20 a of the upper-arm elements UCPand VCP and the lower-arm elements UCN and VCN are each a MOSFET, thediode 20 b connected in inverse parallel with the transistor 20 a can beimplemented using a parasitic diode internally owned by the MOSFETitself. A parasitic diode is also called “body diode”. Use of aparasitic diode eliminates the need for a separate diode and can thusprovide a reduction in the number of components, thereby leading to acost reduction.

In addition, in the case in which the transistors 20 a of the upper-armelements UCP and VCP and the lower-arm elements UCN and VCN are each aMOSFET, at least one of the upper-arm elements UCP and VCP and thelower-arm elements UCN and VCN may be formed of a wide bandgapsemiconductor such as silicon carbide, a gallium-nitride-based material,or diamond. Examples of wide bandgap semiconductor include siliconcarbide (SiC), gallium nitride (GaN), gallium oxide (Ga₂O₃), diamond,and more.

A wide bandgap semiconductor generally has a higher voltage resistanceand a higher heat resistance than a silicon semiconductor. For thisreason, use of a MOSFET formed of a wide bandgap semiconductor in atleast one of the upper-arm elements UCP and VCP and the lower-armelements UCN and VCN enables benefits of high voltage resistance andhigh heat resistance to be received.

The upper-arm elements UCP and VCP and the lower-arm elements UCN andVCN may also each be configured using, for example, an insulated gatebipolar transistor (IGBT) instead of a MOSFET. Note that any parasiticdiode is not formed in an IGBT, and thus when an IGBT is used, the diode20 b connected in inverse parallel therewith is essential.

In addition, although the upper-arm elements UCP and VCP and thelower-arm elements UCN and VCN in FIG. 2 are configured by equal typesof elements, respectively, the embodiment is not limited to thisconfiguration. For example, the two legs 20A and 20B may each use aswitching element as the upper-arm element, and a diode as the lower-armelement. Alternatively, of the two legs 20A and 20B, any one may useswitching elements as the upper-arm element and the lower-arm element,and the other one may use diodes as the upper-arm element and thelower-arm element.

The upper-arm element UCP and the lower-arm element UCN are connected toeach other at a connection point 22, which is connected to one end ofthe AC power supply 10. The upper-arm element VCP and the lower-armelement VCN are connected to each other at a connection point 23, whichis connected to another end of the AC power supply 10. In therectification unit 20, the connection points 22 and 23 each form an ACterminal.

FIGS. 1 and 2 illustrate a configuration including two legs each havingthe upper-arm element and the lower-arm element connected in series witheach other. This configuration is adapted for the AC power supply 10that is a single-phase power supply. If the AC power supply 10 is athree-phase power supply, the rectification unit 20 would be configuredto be adapted to the three-phase power supply accordingly. Specifically,the configuration is supposed to have three legs each having anupper-arm element and a lower-arm element connected in series with eachother. Note that regardless of whether the AC power supply 10 is asingle-phase power supply or a three-phase power supply, one leg may beconfigured to have multiple pairs of upper-arm and lower-arm elements.

Referring back to FIG. 1 , the description of the load driving device100 will be continued. The DC power generated by the conversion in therectification unit 20 is stored in the smoothing capacitor 30.

One example of the smoothing capacitor 30 is an aluminum electrolyticcapacitor. An aluminum electrolytic capacitor has a high capacitance pervolume, thereby enabling the component size to be reduced relative tothe electricity storage capacitance. For this reason, an aluminumelectrolytic capacitor is suitable for size reduction of the device.Note that a high capacitance may cause a high harmonic current to flowinto the AC power supply 10. If a harmonic current presents someproblem, a film capacitor may be used instead of an aluminumelectrolytic capacitor. A film capacitor generally has a longer lifethan an aluminum electrolytic capacitor. Moreover, for the purpose offurther reduction of the harmonic current and improvement in powerfactor, a reactor may be interposed between the AC power supply 10 andthe smoothing capacitor 30.

The voltage between the DC bus lines 25 a and 25 b is applied to thecontrol power supply generation unit 90. The voltage between the DC buslines 25 a and 25 b will hereinafter be referred to as “bus voltage”case by case. The control power supply generation unit 90 steps down thebus voltage and generates a power supply voltage for operating thecontrol unit 60, the voltage detector 80, and so on.

The bus voltage is applied to the inverter 40. The inverter 40 convertsthe DC power stored in the smoothing capacitor 30 into AC power throughthe DC bus lines 25 a and 25 b and supplies the AC power generated bythe conversion to the permanent magnet synchronous motor 50 that is aload.

The inverter 40 includes, as illustrated in FIG. 3 , a leg 40A having anupper-arm switching element UP and a lower-arm switching element UNconnected in series with each other, a leg 40B having an upper-armswitching element VP and a lower-arm switching element VN connected inseries with each other, and a leg 40C having an upper-arm switchingelement WP and a lower-arm switching element WN connected in series witheach other. The leg 40A, the leg 40B, and the leg 40C are connected inparallel with one another.

FIG. 3 illustrates, by way of example, a case in which the upper-armswitching elements UP, VP, and WP and the lower-arm switching elementsUN, VN, and WN are each a MOSFET. The upper-arm switching element UPincludes a transistor 40 a and a diode 40 b coupled in inverse parallelwith the transistor 40 a. The other ones, i.e., the upper-arm switchingelements VP and WP, and the lower-arm switching elements UN, VN, and WNare each configured similarly to the upper-arm switching element UP. Theterm “inverse parallel” as used here means that, similarly to the caseof the rectification unit 20, a first terminal corresponding to a sourceof a MOSFET is connected with an anode of a diode, while a secondterminal corresponding to a drain of the MOSFET is coupled with acathode of the diode.

Note that FIG. 3 illustrates a configuration having three legs eachhaving the upper-arm switching element and the lower-arm switchingelement coupled in series with each other, but the embodiment is notlimited to this configuration. The number of legs may be four or more.In addition, the circuit configuration of FIGS. 1 and 3 is adapted forthe three-phase permanent magnet synchronous motor 50, which is anexample of the load. In a case in which the load is a single-phasemotor, the inverter 40 is supposed to be configured to be adapted to thesingle-phase motor accordingly. Specifically, the configuration willinclude two legs each having an upper-arm switching element and alower-arm switching element coupled in series with each other. Note thatregardless of whether the load is a single-phase motor or a three-phasemotor, one leg may be configured using multiple pairs of upper-arm andlower-arm switching elements.

In the case in which the transistors 40 a of the upper-arm switchingelements UP, VP, and WP and the lower-arm switching elements UN, VN, andWN are each a MOSFET, the diode 40 b coupled in inverse parallel withthe transistor 40 a can be implemented using a parasitic diodeinternally owned by the MOSFET itself. Use of the parasitic diodeeliminates the need for a separate diode, so that the number ofcomponents can be reduced thereby to lead to a cost reduction.

In addition, in the case in which the transistors 40 a of the upper-armswitching elements UP, VP, and WP and the lower-arm switching elementsUN, VN, and WN are each a MOSFET, at least one of the upper-armswitching elements UP, VP, and WP and the lower-arm switching elementsUN, VN, and WN may be made from a wide bandgap semiconductor such assilicon carbide, a gallium-nitride-based material, or diamond.

A wide bandgap semiconductor generally has higher voltage resistance andhigher heat resistance than a silicon semiconductor. For this reason,use of a MOSFET formed of a wide bandgap semiconductor in at least oneof the upper-arm switching elements UP, VP, and WP and the lower-armswitching elements UN, VN, and WN enables benefits of high voltageresistance and high heat resistance to be received.

The upper-arm switching element UP and the lower-arm switching elementUN are connected to each other at a connection point 42, and theconnection point 42 is connected to a first phase (e.g., U phase) lineof the permanent magnet synchronous motor 50. The upper-arm switchingelement VP and the lower-arm switching element VN are connected to eachother at a connection point 43, and the connection point 43 is connectedto a second phase (e.g., V phase) line of the permanent magnetsynchronous motor 50. The upper-arm switching element WP and thelower-arm switching element WN are connected to each other at aconnection point 44, and the connection point 44 is connected to a thirdphase (e.g., W phase) line of the permanent magnet synchronous motor 50.In the inverter 40, the connection points 42, 43, and 44 each form an ACterminal.

Referring back to FIG. 1 , the description of the load driving device100 will be continued. The permanent magnet synchronous motor 50 isdriven by electric power supplied from the inverter 40. The permanentmagnet synchronous motor 50 is one example of load. Any motor configuredto generate regenerative electric power can be the load described in thepresent embodiment.

A configuration and general operation of a control system primarilyincluding the control unit 60 configured to control the inverter 40 willnext be described with reference to FIGS. 1 and 4 . FIG. 4 is a blockdiagram illustrating a detailed configuration of the control unit 60illustrated in FIG. 1 .

The voltage detector 80 detects the bus voltage. In the configuration ofFIG. 1 , the bus voltage is an input voltage to the inverter 40 and alsocorresponds to the voltage across the smoothing capacitor 30. Thevoltage across the smoothing capacitor 30 will hereinafter be referredto as “capacitor voltage” case by case. The voltage detector 80 detectsthe capacitor voltage, and the detected value V_(dc) of the detectedcapacitor voltage is inputted to the control unit 60.

The current detector 82 detects an electric phase current flowing ineach of the phase lines between the inverter 40 and the permanent magnetsynchronous motor 50. This phase current will hereinafter be referred toas “motor current” case by case. The current detector 82 detects a motorcurrent, and detected values i_(u), i_(v), and i_(w) of the detectedmotor current are inputted to the control unit 60.

The control unit 60 generates signals for operating or stopping theoperation of the inverter 40 based on the detected value V_(dc) of thecapacitor voltage and the detected values i_(u), i_(v), and i_(w) of themotor current. FIG. 1 indicates these signals as “CS”. Specifically, thesignals CS are each a pulse width modulation (PWM) signal for performingpower running control on the permanent magnet synchronous motor 50, or astop signal for stopping rotation of the permanent magnet synchronousmotor 50.

The term “power running” as used herein refers to a state in which poweris being supplied from the inverter 40 to the permanent magnetsynchronous motor 50, and the term “power running control” as usedherein refers to control to place the permanent magnet synchronous motor50 into a power running mode. For example, during acceleration of themotor, the rotational speed and the torque have the same sign, by whichthe motor is in a power running mode. An antonym of the term “powerrunning” is “regeneration”. The term regeneration as used herein refersto a state in which rotational energy held in the permanent magnetsynchronous motor 50 is flowing into the inverter 40. For example,during deceleration of the motor, the rotational speed and the torquehave opposite signs, by which the motor is in a regeneration mode.

Note that in FIG. 1 , a configuration is adapted, in which the currentdetector 82 detects the phase currents flowing between the inverter 40and the permanent magnet synchronous motor 50, but the embodiment is notlimited to this configuration. It is possible to adopt a configurationsuch that resistors are provided between the lower-arm switchingelements of the inverter 40 and the DC bus line 25 b on the lowerpotential side, and the currents are detected by measurement of thevoltage across each of the resistors.

The control unit 60 includes, as illustrated in FIG. 4 , a firstcoordinate conversion unit 61, a motor speed estimation unit 62, a motorcontrol unit 63, an integrator 64, a second coordinate conversion unit65, a drive signal generation unit 66, and a voltage drop preventioncontrol unit 67.

The first coordinate conversion unit 61 calculates d-q axis currentsi_(d_m) and i_(q_m) based on the detected values i_(u), i_(v), and i_(w)of the motor current and an estimated magnetic pole position valueθ_(me) generated by the integrator 64 described later. Specifically, thefirst coordinate conversion unit 61 converts the detected values i_(u),i_(v), and i_(w) that are current values represented in the UVWcoordinate system into current values represented in the d-q coordinatesystem using the estimated magnetic pole position value θ_(me). Thecurrent values obtained by the conversion are outputted to the motorspeed estimation unit 62 and to the motor control unit 63, as the d-qaxis currents i_(d_m) and i_(q_m).

The motor speed estimation unit 62 estimates an estimated rotationalspeed value ω_(me) of the permanent magnet synchronous motor 50 based onthe d-q axis currents i_(d_m) and i_(q_m) and d-q axis voltage commandvalues v_(d)* and v_(q)*. The d-q axis voltage command values v_(d)* andv_(q)* are voltage command values on the d-q axes, generated by themotor control unit 63 described later.

The integrator 64 calculates the estimated magnetic pole position valueθ_(me) of the permanent magnet synchronous motor 50 based on theestimated rotational speed value ω_(me). The estimated magnetic poleposition value θ_(me) is calculated by integrating the estimatedrotational speed value ω_(me) in the integrator 64.

The estimated rotational speed value ω_(me) and the estimated magneticpole position value θ_(me) can be estimated using a publicly knowntechnique. For example, details thereof is described in Japanese PatentNo. 4672236, and detailed description thereof will therefore be omittedherein. Note that although the estimated rotational speed value ω_(me)and the estimated magnetic pole position value θ_(me) are estimated inthe control unit 60 in the present embodiment, any technique may be usedas long as it can estimate or detect the rotational speed and themagnetic pole position. In addition, although the estimated rotationalspeed value ω_(me) and the estimated magnetic pole position value θ_(me)are estimated using the d-q axis currents i_(d_m) and i_(q_m) and thed-q axis voltage command values v_(d)* and v_(q)* in the presentembodiment, information described herein may be omitted or informationnot described herein may be used as long as the estimated rotationalspeed value ω_(me) and the estimated magnetic pole position value θ_(me)can be estimated.

The motor control unit 63 calculates the d-q axis voltage command valuesv_(d)* and v_(q)* based on the d-q axis currents i_(d_m) and i_(q_m) andthe estimated rotational speed value ω_(me). Then, the second coordinateconversion unit 65 calculates voltage command values v_(u)*, v_(v)*, andv_(w)* based on the d-q axis voltage command values v_(d)* and v_(q)*and the estimated magnetic pole position value θ_(me). Specifically, thesecond coordinate conversion unit 65 converts the d-q axis voltagecommand values v_(d)* and v_(q)* that are voltage command values on thed-q axes into the voltage command values v_(u)*, v_(v)*, and v_(w)*represented in the UVW coordinate system using the estimated magneticpole position value θ_(me), and outputs the voltage command valuesv_(u)*, v_(v)*, and v_(w)* obtained by the conversion to the drivesignal generation unit 66.

The drive signal generation unit 66 generates a drive signal DS based onthe voltage command values v_(u)*, v_(v)*, and v_(w)* and the detectedvalue V_(dc) of the capacitor voltage. The drive signals DS are signalsfor driving the switching elements of the inverter 40. The inverter 40is controlled by the drive signals DS and applies a desired voltage tothe permanent magnet synchronous motor 50. Note that what is widely usedfor the voltage command values v_(u)*, v_(v)*, and v_(w)* is generally awaveform on which some sine wave or third-order harmonic issuperimposed, but any technique may be used as long as it can drive thepermanent magnet synchronous motor 50.

The drive signal DS is inputted to the voltage drop prevention controlunit 67 together with the detected value V_(dc) of the capacitor voltageand a threshold voltage V_(th). The voltage drop prevention control unit67 performs voltage drop prevention control for preventing the voltageacross the smoothing capacitor 30 from becoming a negative voltage. Whenthe voltage drop prevention control not to be performed, the voltagedrop prevention control unit 67 outputs the drive signal DS as thesignal CS as it is. Alternatively, when the voltage drop preventioncontrol is to be performed, the stop signal described above is generatedbased on the detected value V_(dc) of the capacitor voltage and thethreshold voltage V_(th). That is, in the voltage drop preventioncontrol unit 67, the signal CS corresponds to the stop signal when thevoltage drop prevention control is to be carried out, and the stopsignal causes the switching elements of the inverter 40 to be driven.Note that the voltage drop prevention control will be described indetail later.

FIG. 5 is a block diagram illustrating an example of a hardwareconfiguration by which the functionality of the control system of thecontrol unit 60 in the first embodiment is implemented. In addition,FIG. 6 is a block diagram illustrating another example of a hardwareconfiguration by which the functionality of the control system of thecontrol unit 60 in the first embodiment is implemented.

To implement all or some of the functionalities of the control system ofthe control unit 60 in the first embodiment, it is possible to adopt aconfiguration, as illustrated in FIG. 5 , including a processor 200 thatperforms computation, a memory 202 in which a program or programs to beread by the processor 200 are stored, and an interface 204 that inputsand outputs a signal.

The processor 200 may be computing means such as a computing device, amicroprocessor, a microcomputer, a central processing unit (CPU), or adigital signal processor (DSP). In addition, the memory 202 can be, forexample, a non-volatile or volatile semiconductor memory such as arandom access memory (RAM), a read-only memory (ROM), a flash memory, anerasable programmable ROM (EPROM), or an electrically EPROM (EEPROM)(registered trademark); a magnetic disk, a flexible disk, an opticaldisk, a compact disc, a MiniDisc, or a digital versatile disc (DVD).

The memory 202 has stored therein a program or programs for executingall or some of the functionalities of the control system in the controlunit 60. The processor 200 transfers and receives necessary informationvia the interface 204, and executes a program stored in the memory 202thereby to control the inverter 40.

In addition, the processor 200 and the memory 202 illustrated in FIG. 5may be replaced with a processing circuit 203 as illustrated in FIG. 6 .The processing circuit 203 corresponds to single circuit, a compositecircuit, an application specific integrated circuit (ASIC), afield-programmable gate array (FPGA), or any combination thereof. Theprocessing circuit 203 may be configured using an electric circuitelement or the like, such as an analog circuit or a digital circuitinstead.

An operation of the load driving device 100 when the supply of power tothe load driving device 100 is cut off because of power outage of the ACpower supply 10 or the like will next be described. An interruption ofthe supply of power to the load driving device 100 is hereinafterreferred to as “loss of power” or the like. In addition, an occasionwhere the supply of power to the load driving device 100 is resumedafter the loss of power is referred to as “recovery of power” or thelike.

FIG. 7 is a time chart used for describing an operation at the time ofloss of power in the load driving device 100 of FIG. 1 . FIG. 7illustrates a situation in which the inverter 40 continues its operationeven if a capacitor voltage is dropped at the time of loss of power. Adrop of the capacitor voltage causes the voltage applicable to thepermanent magnet synchronous motor 50 to be lowered, thereby causing therotational speed of the permanent magnet synchronous motor 50 to bedecreased. Meanwhile, when the inverter 40 continues its operation untilthe capacitor voltage becomes near zero in a state where the permanentmagnet synchronous motor 50 is rotating even during a decrease in therotational speed of the permanent magnet synchronous motor 50, aphenomenon occurs in which, as illustrated, the capacitor voltage turnsinto a negative voltage. The condition of a negative voltage of thecapacitor voltage disappears in stoppage of the operation of theinverter 40, and recovery of power of the AC power supply 10 causes theinverter 40 to be started up, and causes the permanent magnetsynchronous motor 50 to rotate again.

FIG. 8 is a diagram illustrating various kinds of waveform examples whenthe operation illustrated in FIG. 7 is carried out. FIG. 9 is a partialenlarged view of a section indicated by a double-headed arrow in FIG. 8. In FIGS. 8 and 9 , a broken line represents the capacitor voltage, adashed-and-dotted line represents the motor current flowing in the firstphase (e.g., U phase) line, and a solid line represents a phase-to-phasevoltage, i.e., a voltage between the first phase (e.g., U phase) lineand the second phase (e.g., V phase) line. In FIG. 9 , a phenomenon isoccurring in which the capacitor voltage turns into a negative voltageas illustrated in a portion encircled by an ellipse.

A mechanism of occurrence of a negative voltage will next be describedwith reference to FIG. 10 . FIG. 10 is a diagram used for describing amechanism of occurrence of a negative voltage, which is consideredproblematic in the first embodiment.

In FIG. 10 , in the event of a loss of power supply, the supply of powerto the smoothing capacitor 30 is cut off, and thereby if power issupplied to the permanent magnet synchronous motor 50, the capacitorvoltage is decreased. The permanent magnet synchronous motor 50continues its rotation by inertia even in this state, and therefore aregenerative current flows therein via the inverter 40. Among varioustypes of motors, a permanent magnet synchronous motor continues toconstantly generate an induced voltage to the motor terminals evenwithout being supplied with power from the outside as long as the rotorrotates, due to the action of magnetic flux of a permanent magnetincorporated in the rotor.

FIG. 10 illustrates a state in which the MOSFET of the upper-armswitching element UP of the U phase, the MOSFET of the upper-armswitching element VP of the V phase, and the MOSFET of the lower-armswitching element WN of the W phase are turned on. In this case, a firstcurrent 72 represented by the solid line in FIG. 10 and a second current74 represented by the broken line flow. The first current 72 a currentflowing from the permanent magnet synchronous motor 50 sequentiallythrough the MOSFET of the upper-arm switching element VP and the MOSFETof the upper-arm switching element UP and returning to the permanentmagnet synchronous motor 50. In addition, the second current 74 is acurrent flowing through a different path from the path of the firstcurrent 72, which is a current flowing from the permanent magnetsynchronous motor 50 sequentially through the MOSFET of the lower-armswitching element WN and the diode of the lower-arm switching element UNand returning to the permanent magnet synchronous motor 50.

The first current 72 is a current flowing from the drain to the sourcein the MOSFET of the upper-arm switching element UP. Thus, a voltagedrop Vsw across the upper-arm switching element UP with reference to thesource that is the first terminal thereof has a positive value. Notethat the present embodiment assumes that a typical SiC-MOSFET is used,and that the forward voltage drop has a value of “0.1 V”.

In addition, the second current 74 is a current flowing from the anodeto the cathode in the diode of the lower-arm switching element UN. Thus,a voltage drop Vdi across the lower-arm switching element UN withreference to the source of the MOSFET that is a first terminal thereofhas a negative value. Note that in the case of SiC, the voltage drop hasa high value since a bandgap thereof is three times or more greater thanSi. The present embodiment hereinafter assumes that the forward voltagedrop of the diode in a typical SiC-MOSFET has a value of “4.0 V”.

According to the foregoing operation, the inverter 40 generates avoltage of 0.1+(−4.0)=−3.9 V and applies this voltage between both endsof the smoothing capacitor 30. This may cause a negative voltage about−4 V to be generated between both ends of the smoothing capacitor 30 ineach repetition of power cut.

Although the foregoing description has been directed to the voltage dropin the leg 40A consisting of the upper-arm switching element UP and thelower-arm switching element UN, a similar negative voltage also occursin the leg 40B or 40C.

In addition, the foregoing description has been given for a point thatthe voltage drop Vsw occurring in the MOSFET of the upper-arm switchingelement UP and the voltage drop Vdi occurring in the diode of thelower-arm switching element UN cause the negative voltage to arise, butan opposite situation may occur. That is, the voltage drop Vdi occurringin the diode of the upper-arm switching element UP and the voltage dropVsw occurring in the MOSFET of the lower-arm switching element UN maycause the negative voltage to arise.

A negative voltage caused by an operation of the rectification unit 20when the AC power supply 10 is subjected to resumption of power willnext be described.

In a situation where the AC power supply 10 is to be resumed, when thevoltage of the AC power supply 10 has, for example, a positive polarity,the diode of the upper-arm element VCP and the diode of the lower-armelement UCN are electrically conducted, thereby a rectified voltagebeing applied between both ends of the smoothing capacitor 30.Alternatively, when the voltage of the AC power supply 10 has, forexample, a negative polarity, the diode of the upper-arm element UCP andthe diode of the lower-arm element VCN are electrically conducted,thereby a rectified voltage being applied between both ends of thesmoothing capacitor 30. Accordingly, when the amount of the forwardvoltage drop across the diode is denoted by “Vf”, the amount of thevoltage drop in the entire rectification unit 20 is “2Vf”.

Assume here that MOSFETs formed of a wide band gap semiconductor areused for the elements of the rectification unit 20. In this case, theamount of the voltage drop in the entire rectification unit 20 withreference to the DC bus line 25 b on the lower potential side is−2Vf=−4.0+(−4.0)=−8.0 V. Accordingly, when the AC power supply 10 isresumed, the rectification unit 20 is not electrically conducted unlessthe AC power supply 10 provides a voltage of 8.0 V or higher. This meansthat a rotation operation of the permanent magnet synchronous motor 50at the time of loss of power may generate a negative voltage up to about−8 V between both ends of the smoothing capacitor 30.

Note that even if, for example, Si-MOSFETs formed of silicon are usedfor the elements of the rectification unit 20, a voltage drop caused bythe parasitic diode of the Si-MOSFET amounts about 2 V. Thus, theapplied voltage not clamped unless the voltage exceeds 4 V thatcorresponds to two parasitic diodes. This may cause a negative voltageabout −4 V to arise between both ends of the smoothing capacitor 30.This value is very high as compared to 1 V that is the amount of thevoltage drop across a rectification diode generally used in therectification unit 20. That is, in the configuration using MOSFETs inthe rectification unit 20, there is a problem in that susceptibility tothe negative voltage becomes higher because the negative voltage ofabout 4 V generated by the inverter 40 employing a wide bandgapsemiconductor is not clamped.

As described above, there is a possibility that a voltage arises betweenboth ends of the smoothing capacitor 30, the voltage being either theamount of the voltage drop in the entire rectification unit 20 (−2Vf) orthe sum (Vsw+Vdi) of a first voltage (Vsw or Vdi) and a second voltage(Vdi or Vsw), whichever is greater. The first voltage is the potentialdifference between the second terminal and the first terminal of theupper-arm switching element in one of the legs of the inverter 40. Thesecond voltage is the potential difference between the second terminaland the first terminal of the lower-arm switching element of that leg.

FIG. 11 is a graph for describing an influence on the smoothingcapacitor 30 illustrated in FIG. 1 in the case of occurrence of anegative voltage. Note that the smoothing capacitor 30 is assumed to bean electrolytic capacitor. In FIG. 11 , the horizontal axis representsthe applied voltage to the electrolytic capacitor, and the vertical axisrepresents the leakage current flowing in the electrolytic capacitor. Asillustrated in FIG. 11 , an excessive leakage current may be generatedeven when a relatively low negative voltage is applied. Occurrence of anexcessive leakage current causes an increase in the temperature insidethe electrolytic capacitor. Repeated temperature increase of theelectrolytic capacitor may shorten the life of the electrolyticcapacitor.

FIG. 12 is a diagram for describing an influence on the control powersupply generation unit 90 illustrated in FIG. 1 in the case ofoccurrence of a negative voltage. FIG. 12 illustrates a typical circuitconfiguration of the control power supply generation unit 90. Asillustrated in FIG. 12 , a typical control power supply generation unit90 includes a control power supply IC 91, a transformer 92, a diode 93for backflow prevention, a capacitor 94 for smoothing, and a controlcircuit 95 to which a power supply voltage is applied. In addition, thecontrol power supply IC 91 includes a control power supply IC controlunit 91 a and a MOSFET 91 b. A primary winding 92 a of the transformer92 and the MOSFET 91 b are connected in series with each other. The busvoltage is applied between both ends of a series connection of theprimary winding 92 a and the MOSFET 91 b. Then, the stepped-down voltageinduced in a secondary winding 92 b of the transformer 92 is applied tothe capacitor 94.

When the bus voltage is a negative voltage, a positive voltage isapplied to the anode of the parasitic diode formed in the MOSFET 91 b.In this situation, the parasitic diode of the MOSFET 91 b is conductedupon occurrence of the negative voltage about −4 V described above. In acase in which the control power supply IC 91 is formed of a single-chipsemiconductor device, conduction of electricity through the parasiticdiode may cause the parasitic transistor (not illustrated) formed insidethe control power supply IC 91 to malfunction. Malfunction of theparasitic transistor of the control power supply IC 91 may result infailure to generate the power supply voltage, and in failure to applythe power supply voltage to the control unit 60, thereby making itimpossible to drive the permanent magnet synchronous motor 50.

In the circumstances, the control unit 60 of the first embodimentperforms the voltage drop prevention control described above. FIG. 13 isa flowchart used for describing an operation according to the voltagedrop prevention control in the first embodiment. The process of theflowchart of FIG. 13 is invoked in the event of a loss of power. Thevoltage drop prevention control unit 67 performs the following controlaccording to the flowchart of FIG. 13 .

First of all, the comparator 66 compares the detected value V_(dc) ofthe capacitor voltage with the threshold voltage V_(th) (step S11). Ifthe detected value V_(dc) is less than or equal to the threshold voltageV_(th) (Yes at step S11), the voltage drop prevention control unit 67outputs the signal CS that is a stop signal (step S12).

One example of the stop signal is a zero vector. A zero vectorcorresponds to an output signal that causes the windings (notillustrated)) of the permanent magnet synchronous motor 50 to beelectrically short-circuited. An example of the zero vector is a drivesignal that turns on the lower-arm switching elements UN, VN, and WN,and turns off the upper-arm switching elements UP, VP, and WP. Anotherexample of the zero vector is a drive signal that turns off thelower-arm switching elements UN, VN, and WN, and turns on the upper-armswitching elements UP, VP, and WP. Note that, instead of using a zerovector, outputting of the drive signal DS may be stopped.

Otherwise, if the detected value V_(dc) exceeds the threshold voltageV_(th) in the flow of FIG. 13 (No at step S11), the voltage dropprevention control unit 67 outputs the drive signal DS generated by thedrive signal generation unit 66 as it is (step S13).

Note that, at step S11 described above, the case where the detectedvalue V_(dc) is equal to the threshold voltage V_(th) is determined tobe the “Yes” case, but it may be determined to be the “No” case. Thatis, if the detected value V_(dc) is equal to the threshold voltageV_(th), the process of step S13 may be performed.

FIG. 14 is a block diagram illustrating a configuration example of thevoltage drop prevention control unit 67 in the first embodiment. FIG. 14illustrates a configuration example in a case in which the voltage dropprevention control the first embodiment is carried out in hardware. Thevoltage drop prevention control unit 67 includes, as illustrated in FIG.14 , a comparator 68 and a stop signal generation unit 69.

An operation of the voltage drop prevention control unit 67 illustratedin FIG. 14 will next be described with reference to the drawings ofFIGS. 14 to 18 . FIG. 15 is a first time chart illustrating an operationof the voltage drop prevention control unit 67 illustrated in FIG. 14 .FIG. 16 is a second time chart illustrating an operation of the voltagedrop prevention control unit 67 illustrated in FIG. 14 . FIG. 17 is adiagram used for describing current paths when the voltage dropprevention control unit 67 performs the operation illustrated in FIG. 15. FIG. 18 is a diagram used for describing current paths when thevoltage drop prevention control unit 67 performs the operationillustrated in FIG. 16 .

In FIG. 14 , the comparator 68 receives the detected value V_(dc) of thecapacitor voltage and the threshold voltage V_(th). The comparator 68outputs a detection signal Sp when the detected value V_(dc) is lessthan or equal to the threshold voltage V_(th). When the detection signalSp is inputted, the stop signal generation unit 69 outputs, as thesignal CS, the stop signal generated inside the stop signal generationunit 69. Alternatively, when no detection signal Sp is inputted, thestop signal generation unit 69 outputs the drive signal CS inputted, asthe signal CS as it is.

Note that the above description is directed to a point that thedetection signal Sp is outputted when the detected value V_(dc) is lessthan or equal to the threshold voltage V_(th), but the present inventionis not limited to this example. Instead of this example, use may be madeof a signal for outputting a “logical 1” when the detected value V_(dc)is less than or equal to the threshold voltage V_(th), and outputting a“logical 0” when the detected value V_(dc) exceeds the threshold voltageV_(th), for example.

FIGS. 15 and 16 each illustrate a situation in which the capacitorvoltage drops and falls below the threshold voltage V_(th) t time t1.

In FIG. 15 , the hatched section until time t1 is a PWM signal outputperiod, and the section after time t1 is a no-output period. During thePWM signal output period, the drive signal DS causes any of theupper-arm switching elements UP, VP, and WP and the lower-arm switchingelements UN, VN, and WN to be in operation. In contrast, during theno-output period, none of the upper-arm switching elements UP, VP, andWP and the lower-arm switching elements UN, VN, and WN have theirswitching operations stopped.

In addition, in FIG. 16 , the hatched section until time t1 is a PWMsignal output period, and the section after time t1 is a zero-vectoroutput period. During the PWM signal output period, the drive signal DScauses any of the upper-arm switching elements UP, VP, and WP and thelower-arm switching elements UN, VN, and WN to be in operation. Incontrast, during the zero-vector output period, the upper-arm switchingelements UP, VP, and WP are in off operation, and the lower-armswitching elements UN, VN, and WN are in on operation.

FIG. 17 illustrates an example of current paths when the voltage dropprevention control unit 67 performs the operation illustrated in FIG. 15. As illustrated in FIG. 17 , one of the currents flowing from theW-phase winding (not illustrated) of the permanent magnet synchronousmotor 50 flows sequentially through the diode of the upper-arm switchingelement WP, the smoothing capacitor 30, and the diode of the lower-armswitching element UN and returns to the permanent magnet synchronousmotor 50. In addition, another one of the currents flowing from theW-phase winding (not illustrated) of the permanent magnet synchronousmotor 50 flows sequentially through the MOSFET of the upper-armswitching element WP, the smoothing capacitor 30, and the diode of thelower-arm switching element VN and returns to the permanent magnetsynchronous motor 50. Both of these currents are currents that chargethe smoothing capacitor 30 to a positive voltage. Thus, by the voltagedrop prevention control unit 67 performing the voltage drop preventioncontrol illustrated in FIG. 15 , it is possible to reliably prevent thesmoothing capacitor 30 from undergoing a negative voltage.

FIG. 18 illustrates an example of current paths when the voltage dropprevention control unit 67 performs the operation illustrated in FIG. 16. As illustrated in FIG. 18 , one of the currents flowing from theW-phase winding (not illustrated) of the permanent magnet synchronousmotor 50 flows sequentially through the MOSFET of the lower-armswitching element WN and the MOSFET of the lower-arm switching elementUN and returns to the permanent magnet synchronous motor 50. Inaddition, another one of the currents flowing from the W-phase winding(not illustrated) of the permanent magnet synchronous motor 50 flowssequentially through the MOSFET of the lower-arm switching element WNand the MOSFET of the lower-arm switching element VN and returns to thepermanent magnet synchronous motor 50. When these currents flow, thesecond voltage Vdi due to the forward voltage drop component isgenerated in the MOSFET of the lower-arm switching element UN and theMOSFET of the lower-arm switching element VN. However, the secondvoltage Vdi generated in each of the MOSFETs of the lower-arm switchingelement UN and the lower-arm switching element VN is electricallyseparated from the smoothing capacitor 30, and therefore does not affectthe voltage across the smoothing capacitor 30.

Note that an example of FIG. 16 illustrates that in the zero-vectoroutput period, the upper-arm switching elements UP, VP, and WP areturned off and the lower-arm switching elements UN, VN, and WN areturned on, but the operations of the upper-arm switching elements UP,VP, and WP and the operations of the lower-arm switching elements UN,VN, and WN may be made in opposite on/off pattern to the example. Thatis, in the zero-vector output period, it is possible that the upper-armswitching elements UP, VP, and WP are turned on and the lower-armswitching elements UN, VN, and WN are turned off.

Note that it is sufficient that the threshold voltage V_(th) be set to avoltage value that does not cause the smoothing capacitor 30 to have anegative voltage, and any such value may be set as the threshold voltageV_(th). Nevertheless, a suitable value is preferably set as thethreshold voltage V_(th) in consideration of the means for detecting theloss of power. For example, when the AC power supply is a utility powersupply at 50 Hz, one cycle as the power supply cycle is 20 ms. Forexample, in a case in which the means for detecting the loss of power isof a type that operates to detect a zero crossing point in the powersupply cycle, detection of loss of power may be delayed, before which adrive signal based on the power running control may be applied. For thisreason, it is necessary that the threshold voltage V_(th) be set takinginto consideration the delay time of detection of loss of power and/orthe like. It is therefore important that the threshold voltage V_(th) beset to cause the power running control to be stopped in the state inwhich the voltage across the smoothing capacitor is higher than the sum(Vsw+Vdi) of the first voltage (Vsw) and the second voltage (Vdi). Notethat the first voltage (Vsw) is a voltage generated across the upper-armswitching element of one of the legs, and the second voltage (Vdi) is avoltage generated across the lower-arm switching element of the same lagas the leg in which the first voltage (Vsw) is generated.

As described above, the load driving device 100 according to the firstembodiment stops performing the power running control in the state inwhich the voltage across the smoothing capacitor 30 is higher than thesum (Vsw+Vdi) of the first voltage (Vsw) and the second voltage (Vdi),and can therefore reliably prevent the voltage across the smoothingcapacitor from becoming a negative voltage without including anadditional component.

Note that, as described above, a negative voltage of the smoothingcapacitor is noticeable when a switching element of the rectificationunit 20 or a switching element of the inverter 40 is formed of a widebandgap semiconductor. Therefore, the voltage drop prevention control inthe first embodiment is particularly advantageous in a case in which aswitching element of the rectification unit 20 or the inverter 40 isformed of a wide bandgap semiconductor.

Second Embodiment

FIG. 19 is a circuit diagram illustrating a configuration example of aload driving device 100A according to a second embodiment. The loaddriving device 100A illustrated in FIG. 19 has a configuration based onan assumption that it is used as an outdoor unit of an air conditioner.Specifically, it is assumed herein that a permanent magnet synchronousmotor 50 is applied to a compressor motor and a second permanent magnetsynchronous motor 55 is applied to a fan motor.

The load driving device 100A according to the second embodimentillustrated in FIG. 19 further includes, in addition to theconfiguration of the load driving device 100 according to the firstembodiment illustrated in FIG. 1 , a second inverter 45 that supplies ACpower to the second permanent magnet synchronous motor 55, and a secondcurrent detector 84 that detects a second motor current flowing betweenthe second inverter 45 and the second permanent magnet synchronous motor55. DC terminals of the second inverter 45 are connected to the DC buslines 25 a and 25 b. Accordingly, shown is a configuration that the DCvoltage across the smoothing capacitor 30 is applied to the inverter 40and to the second inverter 45 via the shared DC bus lines 25 a and 25 b.The second current detector 84 detects a motor current, and detectedvalues i_(uf), i_(vf), and i_(wf) of the detected motor current areinputted to the control unit 60. The control unit 60 generates signalsCS2 for either operating or stopping the second inverter 45 based on thedetected value V_(dc) of the capacitor voltage and the detected valuesi_(uf), i_(vf), and i_(wf) of the second motor current, and outputs thesignals CS2 to the second inverter 45. Note that the other componentsare identical or equivalent to the corresponding components of the firstembodiment illustrated in FIG. 1 . The identical or equivalentcomponents are designated by the same reference characters, andredundant description will be omitted.

In an air conditioner, the compressor motor consumes several hundredswatts to several kilowatts of electrical power, whereas the fan motorconsumes at most several tens watts to a hundred watts of electricalpower. That is, the compressor motor consumes a larger amount ofelectrical power than the fan motor. For this reason, when loss of powerarises, a prolonged rotation operation of the compressor motor is morelikely to cause a rapid drop of the voltage across the smoothingcapacitor 30 to thereby generate the negative voltage described in thefirst embodiment. Therefore, a preferred embodiment is in line with amanner that the compressor motor is preferentially stopped before thefan motor. Such stopping of the compressor motor before stopping of thefan motor can reliably prevent occurrence of a negative voltage.

In addition, the fan motor has a larger moment of inertia than thecompressor motor. For this reason, the fan motor has a characteristic ofcontinuing its rotation for a long period of time even after the voltageapplication to the inverter 40 and to the second inverter 45 isinterrupted. Thus, even when the compressor motor is antecedentlystopped before the fan motor, the fan motor continues its rotation andheat exchange, thereby enabling prevention of a pressure increase in therefrigeration cycle relying on which the air conditioner is configured,and thus enabling the air conditioner to be stopped more safely. Asdescribed above, the voltage drop prevention control to stop thecompressor motor antecedent to the fan motor contributes to enhancingthe effectiveness of the control to safely stop the air conditioner, andthereby leads to a more preferred embodiment for an air conditioner.

Note that the configurations described is the foregoing embodiments aremerely examples of a concept of the present invention, and so can eachbe combined with other publicly known techniques and partially omittedand/or modified without departing from the scope of the presentinvention.

The invention claimed is:
 1. A load driving device supplyingalternating-current power to first and second permanent magnetsynchronous motors and driving a load including the first and secondpermanent magnet synchronous motors, the load driving device comprising:a smoothing capacitor; first and second inverters each having at leasttwo legs, each of the legs having an upper-arm switching element and alower-arm switching element connected in series with each other, each ofthe inverters converting direct-current power stored in the smoothingcapacitor into the alternating-current power; and a control unitcontrolling the first and second inverters, wherein the first inverterdrives a first load including the first permanent magnet synchronousmotor, the second inverter drives a second load including the secondpermanent magnet synchronous motor, the second load has a moment ofinertia higher than a moment of inertia of the first load, the controlunit performs voltage drop prevention control to stop power runningcontrol on the first and second loads in a state in which the voltageacross the smoothing capacitor is higher than a sum of a first voltageand a second voltage, the first voltage being an electrical potentialdifference between a second terminal of the upper-arm switching elementand a first terminal of the upper-arm switching element with referenceto the first terminal of the upper-arm switching element, the secondvoltage being an electrical potential difference between a secondterminal of the lower-arm switching element and a first terminal of thelower-arm switching element with reference to the first terminal of thelower-arm switching element in the same leg as a leg of the upper-armswitching element, stops the power running control on the first andsecond loads when the voltage across the smoothing capacitor is lowerthan a threshold voltage, and stops rotation of the first permanentmagnet synchronous motor before the second permanent magnet synchronousmotor.
 2. The load driving device according to claim 1, wherein thefirst voltage is a voltage drop caused by a current flowing in aparasitic diode of the upper-arm switching element or a voltage dropcaused by a current flowing in a diode connected in parallel with theupper-arm switching element, and the second voltage is a voltage dropcaused by a current flowing in a transistor of the lower-arm switchingelement in the same leg as a leg of the upper-arm switching element. 3.The load driving device according to claim 1, wherein the first voltageis a voltage drop caused by a current flowing in a transistor of theupper-arm switching element, and the second voltage is a voltage dropcaused by a current flowing in a parasitic diode of the lower-armswitching element or a voltage drop caused by a current flowing in adiode connected in parallel with the lower-arm switching element, in thesame leg as a leg of the upper-arm switching element.
 4. The loaddriving device according to claim 1, wherein switching elementsconstituting an inverter that is at least one of the first and secondinverters are metal-oxide semiconductor field-effect transistors, and atleast one of the metal-oxide semiconductor field-effect transistors isformed of a wide bandgap semiconductor.
 5. The load driving deviceaccording to claim 1, comprising: a rectification unit converting analternating-current voltage outputted from an alternating-current powersupply into a direct-current voltage and applying the direct-currentvoltage to the smoothing capacitor, wherein the rectification unit hastwo or more legs, an upper-arm element and a lower-arm element of eachof which are connected in series with each other, and the upper-armelement of each of the legs is configured by a switching element or thelower-arm element of each of the legs is configured by a switchingelement, or the upper-arm element and the lower-arm element of at leastone of the legs are each configured by a switching element.
 6. The loaddriving device according to claim 5, wherein at least one of switchingelements constituting the rectification unit is formed of a wide bandgapsemi conductor.
 7. A refrigeration cycle applicable apparatus equippedwith the load driving device according to claim
 1. 8. An air conditionerin which the refrigeration cycle applicable apparatus according to claim7 is set, wherein the first permanent magnet synchronous motor drives acompressor, and the second permanent magnet synchronous motor drives ablower.